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NUTRITION OF OYSTERS: THE NATURE OF 
THE SO-CALLED "FATTENING" OF OYSTERS 

By Philip H. Mitchell 

From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXXV, 1915-16 
Document No. 860 : : : : : : : : : : : : : Issued March ij, roiS 




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D« of D. 
APR 20 1918 



NUTRITION OF OYSTERS: THE NATURE OF THE SO-CALLED 
"FATTENING" OF OYSTERS. 

By PHILIP H. MITCHELL. 

Contribution from the United States Fisheries Biological Station, Woods Hole, Mass., and the Biological 

Laboratory of Brown University. 



INTRODUCTION. 

The term "fat" as applied to oysters refers in a popular sense to their appearance. 
When in that condition the meats look plump and have in the body portions a milky 
appearance not unlike emulsified fat. The juice running out of the meats, however, 
shows the opalescence characteristic of glycogen solutions. This, together with the 
fact that glycogen is shown by analysis to be especially abundant in some specimens of 
oysters and to vary greatly in different specimens, suggest the possibility that glycogen 
ma)^ be the chief if not the only substance increased in oysters when they become 
"fat." 

In a previous paper" it was shown that glycogen shows seasonal variations in 
oysters and that an increase of glycogen accompanies favorable feeding conditions. 
It was also shown that glycogen storage not . only accompanied the nonnal feeding 
process, but could occur as the result of assimilation of sugar in solution in the water 
utilized by oysters. 

This paper presents evidence in the fonn of chemical analyses'* of oysters in varying 
nutritive conditions to show that the amount of glycogen present is the only material 
which marks a notable difference between "fat" and "lean" oysters. 

VARIATIONS OF PROTEIN IN THE OYSTER COMPARED WITH THOSE OF GLYCOGEN. 

The percentage of glycogen or protein in whole oyster meats is not a useful index 
as to their nutritive condition or their value as human food. The great variations in the 
proportion of water present in oysters are obvious causes of apparent variations in 
other constituents when figured as percentages. It goes almost without saying that 
the results of analyses must be expressed in terms of percentage of dry substance. An 
equally important variable is the salt content. Ash determinations made under com- 
paraljle conditions have shown in these analyses variations from 14 to 37 per cent of the 
dried weight. It is therefore necessary in comparing determinations of glycogen, 

o "Nutrition of oysters: Glycogen formation and storage." Bull. Bureau of Fisheries, vol. xxtcv, 1915-16. pp. 151-161. 
t" Part of the analytical work presented in tliis paper was done by .A.. E. Barnard. 

27S99°— l.S 479 



48o 



BULLETIN OF THE BUREAU OF FISHERIES. 



protein, etc., in oysters to express results in terms of percentage content of the ash-free 
solids. Glycogen and nitrogen, the latter to be used as an index of the amount of 
protein, were determined in many specimens of oysters of varying nutritive conditions. 
Some oysters were analyzed immediately after removal from their natural habitat, 
others after treatment in various artificial ways. 

The results of a series of analyses are given in Table i. The arrangement is in 
decreasing order of the amounts of glycogen in the ash-free solids. 

Table i. — Comp.\rison of the Glycogen and NriROGEN' Content or Oyster Meats. 





Dried meat 




Ash-free solids. 




Dried meats. 


Ash-free solids. 


Experi- 
ment 






























ment 












No. 


Glycogen 


Nitrogen 


Ash con- 


Glycotreu 


Nitrogen 


No. 


Glycogen 


Nitrogen 


Ash con- 


Glycogen 


Nitrogen 




content. 


content. 


tent. 


content. 


content. 




content. 


content. 


tent. 


content. 


couteut. 




Prr cent. 


Per cent. 


Ptr cent. 


Per iiill- 


Per cent. 




Per cent. 


Per cent. 


Per cent. 


Per cent. 


Per cent. 


I 


iS-SS 


7-31 


17.40 


2... 46 


8.86 


17 


7-44 


8.80 


28.64 


10- 45 


12-33 


3 


17- 6l 


7.98 


19. 10 


21.76 


9.87 


18 


6- 19 


7-86 


35- 70 


9- 63 


12.22 




I.'. 51 


8.61 


21. 20 


15- 88 


10.93 


19 


6.07 


7-86 


32-15 


8.94 


II. 61 


4 


lo. 54 


7.92 


29.30 


14-89 


11.20 


20 


6.17 


8-21 


30. 22 


8-8j 


II. 78 


S 


lo. 51 


8.04 


28.94 


14-79 


II. 20 


21 


5.07 


8.08 


33-52 


7-63 


12.17 


6 


g. yo 


7.50 


33.00 


14. 76 


11. 20 


22 


4.85 


8.05 


34- 20 


7-38 


12. 22 


7 


10. s6 


7.96 


27-78 


14. 61 


11.03 


23 


5-37 


9- 03 


26. 40 


7-29 


12. 26 


S 


9.18 


7- 17 


35-42 


14.22 


11. 10 


24 


5-02 


8. 19 


30.60 


7-25 


11.82 




10.65 


8. 8i 


23-30 


13-91 


II. 50 


= 5 


4- 55 


7.46 


35-75 


7-09 


11.62 


lO 


10.80 


y.jS 


21.97 


13-85 


11. 90 


26 


4-03 


7-58 


37- 72 


6.49 


12.17 


11 


8.69 


7- 13 


35- 60 


IJ- 51 


11.08 


27 


4- 20 


S. 04 


34- 20 


6.40 


12. 22 


12 


10. 76 


9. 20 


19-50 


13- 38 


11.42 


28 


4-34 


8. ;6 


lO- lO 


6.23 


12-57 




S. 76 


7.96 


27-55 


12- 10 


11.00 


29 


3-78 


7-73 


35.60 


5-8S 


12.02 


■4 


9. =6 


9.OJ 


21. 38 


11. 77 


11.48 


30 


3-51 


7-48 


3-60 


5-64 


12.01 


IS 


7-59 


7-32 


34-00 


11.50 


11. 10 


31 


3-82 


8.76 


31-62 


5- 59 


12. 11 


i6 


7-8S 


8.6s 


29-41 


11. 11 


12.24 


32 


1-93 


8.02 


36-75 


3- 05 


12.69 



Examination of Table i shows that as the percentage of glycogen in the ash-free 
solids decreases the percentage of nitrogen, similarly computed, tends to increase. 
There is not a regular mathematical relationship between the two sets of figures, but 
many of the irregularities would fall within the limits of experimental errors. At 
any rate, the series shows strikingly that protein, as indicated by nitrogen determina- 
tions, does not increase in oysters as an accompaniment to glycogen storage. 

In spite of their long continued growth, oysters, indeed, have some tendency 
toward nitrogen equilibrium. Like the higher animals, oysters not only store glycogen 
in preference to protein when food is plentiful, but also use glycogen to protect them- 
selves from loss of body protein when food is scarce. Evidence of this is shown by a 
more detailed examination of some of the results recorded in this table. Eleven of 
these results are segregated in Table 2. They were selected because in each case pre- 
vious experiments, recorded in the first paper" of this series, showed that changes in 
glycogen amounting to 10 per cent or more had occurred in periods from 2 to 14 days. 
The various abnormal feeding conditions causing these sudden fluctuations in glycogen 
content are explained in Table 2. 

That the comparatively small variations in the nitrogen percentages in ash-free 
solids are merely due to the glycogen fluctuations can be seen from the computations 
of the percentage of nitrogen figured not only on an ash-free but also glycogen-free 
basis. These results are sufliciently uniform to show that sudden variations in the 
food supply of oysters are not accompanied by changes in their protein content. 

u " Nutrition of oysters: Glycogen fonnatimi and storage." Bull. Bureau uf Fislieries, vol. xx.xv. 1915-16, pi). 151-161. 



NUTRITION OK OYSTERS: NATURK OK THE " K ATTENING " OK OYSTERS. 48 1 



Table 



-Comparison of Glycogen and Nitrogen of Oysters which .Show Si^dden Changes 
IN Glycogen Due to Abnormal Feeding Conditions. 



Extieri- 

nient 
No. 



I'ed dextrose 

do 

Fed chopped seaweeds 

Starved in fdtered water 

Starved iu partly purified water. . 

Fed dextrose 

do 

do 

Ill polluted water 4 days 

Changed from salt to fresh water . 
In polluted water 14 days 



Ash-free solids. 



C'.lycOKfil 
auiltiit. 



Per cent. 

14. 76 
I4-6I 

13- yi 
13-51 
8.94 

8. S3 
J- 63 

6.23 
5-59 
3- OS 



NiUoReu 
content. 



NitroKeu 

ill asli' 
free and 
ulycogen- 
free solids. 



Per cent. 
11. 20 
11-03 
1 1. 10 
II- so 
11.08 

11. 61 
11.78 
12. 17 
12.57 

12. II 
13-69 



Per cent. 

13- M 
12.94 
12.95 

12.81 
12.75 
12.92 

13- l6 

13.40 
12.84 
13. Cy 



Changes in the proportion of protein present, aside from the unifonn increase due to 
growth, no doubt occur in the oyster. An instance is shown by examination of certain 
of these resuUs. Those in Table 3, chosen because they represent analyses made very 
soon after the oysters were taken from their natural habitat, show marked differences 
in their nitrogen content. This is true even when figured on a glycogen-free basis. That 
seasonal changes are responsible for this is indicated by the fact that oysters taken in 
July and August, which include the spawning season, tend to show a higher proportion 
of nitrogen than those taken in November. Further work would be required to give an 
adequate explanation of this, but the suggestion that accumulation of egg and sperm 
materials, together with heightened metabolism of reproductive glands, may l)e the 
explanation is obvious. 



Table 3. 



-Comparison of Glycogen and Nitrogen of Oysters wmcii Had Not Been Surjected 
to Abnormal Experimental Conditions. 



ICxpcrimeiit No. 



4- 
16 
18 



Dale 
when 
taken 
from 
water. 



Nov. 15 

Nov. 15 

Aujj. 20 

Aug. 7 

Aug. 10 

July 7 



Ash-free solids. 



Glycogen 
content. 



Per cent. 
22. 52 
21. 65 
14.89 
II. II 
9- 63 
7-29 



Kitrogeu 
content. 



Per cent. 
8.86 
9.87 
II. 20 
12.24 



Nitrogen 
in ash-free 

and gly- 
cogen-free 
solids. 



■ cent. 
11.46 
12.O0 
13-17 
13.76 
13- 53 
13- 17 



ICxperiinent No 



Date 
when 
takL-n 
from 
water. 



July 27 

26 1 July 29 

=7 ' July 19 

29... July 20 

30 ' July 22 



Ash-free solids. 



Glycogen 
content. 



Per cent. 
7.09 
0.49 
6- 40 
5-86 
5-64 



Nitrogen 
content. 



Nitrogen 
in a;-Ii- 
f rec and 
glycogen- 
free solitK. 



Per cent. 
11.62 



• cevt. 
12-54 
13. 00 
13.03 
12.79 



VARIATIONS OF FAT IN THE OYSTER COMPARED WITH THOSE OF GLYCOGEN. 

The storage of fat in oysters, as detected by ether extraction of the dried meats, was 
also investigated. In the previous paper " the suggestion that fat might be formed from 
dextrose was tentatively made. It was based, however, on only two experiments and 
is not substantiated by the results of 13 analyses reported in Table 4 below. These later 

u Bull,, Bureau of Fisherie'., vol, xxxv, 1915-16. pp. 155-161. 



482 



BULLETIN OF THE BUREAU OF FISHERIES. 



experiments were made with very careful technique. The oyster meats were dried at 
low temperature — some of them in vacuum desiccators — to constant \veight and the 
ether used for extraction was rendered anhydrous by distillation over sodium imme- 
diately before use. The seeming increase of fat reported for one of the earlier experi- 
ments may have been due to the difficulty in maintaining ether in an anhydrous con 
dition in the moist atmosphere of Woods Hole where the analysis was made. The re- 
sults given in Table 4 do not show in the amounts of ether extract obtained any regu- 
larity or any relationship to glycogen. A number of other fat determinations on oysters 
have been made during the progress of this work. These are not included in this table 
because glycogen was not determined on the same specimens. In no case, however, 
did the ether extract amount to more than 3.50 per cent of the dried meats. A series of 
analyses reported by Atwater" gives higher figures, ranging from 6.50 to 10.97 per cent, 
with an average of 8.78 per cent for 34 analyses. .\s those determinations were not 
made with the use of anhydrous ether, they are hardly comparable with the ones reported 
in this investigation. 

Tabi.i! 4.— Comp.\rison op the Glycogen and Fat Content of Ovster Meats. 



!-!xperiliU'nt Nil. 



Gly- 


Fat 
(ether 


Fat ia 


ash-free 


extract) 
iu dried 
meats. 


ash-free 
sohds. 


Per cent. 


Per ceKt. 


Ptr cent. 


22.46 


2.90 


3- SI 


21. ;6 


3.16 


3.91 


14. 22 


2.04 


3-16 


13.31 


I-5I 


2.28 


8.94 


2-93 


4-32 


8. S3 


1.19 


I. 71 



li.xpcrinieiit Ko. 



Gly- 
cogen in 
ash-free 
sohds. 



anl. 
--t>3 
7.33 
6- 40 
6.23 
5-SS 

5 :;9 



Fat 

(ether 

extract) 

in dried 

meals. 



Per cent. 
2' 33 
2.2b 
I. 41 

2.SS 
I. 72 
1.47 

1. ;o 



Fat iu 
ash-free 
soHds. 



Per cent. 



3-44 
2- IS 



The oysters showing the high glycogen content were the ones which presented a 
"fat" appearance. Indeed, the two specimens yielding the highest glycogen figures 
were selected for analysis by practical oystermen and chosen from beds of oysters 
deemed to be in the best marketable condition. The conclusion that glycogen is the real 
nature of the "fat" does not rest alone on the results recorded in the preceding tables. 
During the past two years glycogen determinations have been made on many samples 
of oysters in connection with this work. However, only those for which ash and either 
nitrogen or fat, or both, have been determined also are tabulated here. Of the other 
specimens it has been noticeable that the higher the glycogen the "fatter" the oysters 
appeared. Six samples from Lynnhaven Bay, Va., and Narragansett Bay, R. I., con- 
sidered by the trade to be in good marketable condition, contained glycogen varying 
from 15.5 to 22,8 per cent of the dried weight and from 20 to 27,9 per cent of the ash- 
free solids. 

DISTRIBUTION OF GLYCOGEN. 

The distribution of glycogen in the bodies of oysters of average "fatness" was inves- 
tigated. About 50 oysters were opened innnediately after removal from the water, 
about September 15, when glycogen formation is rapid. All the juice was drained off 

o Atwater. W. O.: "The chemical composition and nutritive value of food fishes and aquatic .invertebrates," Report of 
United St.nes Commission of Fish and Fisheries, iSSS. pp. 679-868. 



NUTRITION OF OYSTERS: NATURE OF THE "FATTENING" OF OYSTERS. 483 

from the shell contents and evaporated to dryness. The gills and mantles were dis- 
sected off from each meat, mixed together, dried, and gromid. vSimilarly, the adductor 
muscle was separated and prepared. The remainder, or body, of the oyster containing 
the liver, digestive system, etc., was dried and ground into one preparation. Glycogen 
determinations on the four parts of the oyster thus obtained are reported in Table 5. 
These show little or no tendency for glycogen to diffuse out into the shell liquor of the 
oyster, and indicate that like higher animals oysters can store glycogen in all tissues but 
more especially in the liver, for the so-called liver is the chief organ in the part designated 
as the body of the oyster. 

T.\BI.E 5. — DiSTRIBUTIO.V OF GLYCOGEN IN OVSTERS. 



Tarts. 



Bods' 

Gills and mantles. 

Muscle 

Oyster liquid 



Gh'coccn 
in dried 

sub- 
stance. 



Per cmt. 

37.60 

12. 69 

8. SI 

(") 



Ash in 

dried sub 

stance. 



Glycocea 
in ash- 
free 
soUds. 



Per crvl. 


Per cent. 


12.71 


.11.61 


18. ,11 


T>- '=^^ 


10.77 


9-53 


71.0 





" Too low to be accurately determined. 

CONCLUSIONS. 

1. Protein and fat do not accumulate in oysters when they atlain the condition 
known as "fat." This is in marked contrast to the accumulation of glvcogcn which 
must be regarded as the chief storage substance for oysters. "Fat " oysters are glycogcu- 
rich oysters. Investigations and practical procedures looking to improvements in mar- 
ketable value of oysters must take into account the importance of those nutritive condi- 
tions favoring glycogen fonnation. 

2. The glycogen storage occurs more or less in all tissues of the oysters but is espe- 
cially prominent in the region of the liver. 



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